In current LTE systems, an LTE-compatible UE (User Equipment) has to be scheduled by the eNB (eNodeB) to send uplink (UL) data transmissions on PUSCH (Physical Uplink Shared Channel). This is realised by the eNB sending a PUSCH transmission request, also known as an uplink (UL) grant, to the UE. This normally follows an indication by the UE that it has uplink data to send in its buffer. Such uplink grants can be dynamic or configured (periodic). However, the UE sends padding transmissions on PUSCH (Physical Uplink Shared Channel) in an uplink grant procedure if it has no UL (uplink) data in its buffer. This is wasteful of UE battery resource and can add to UL interference.
In order to realise “fast uplink access” it has been considered to use pre-scheduling whereby the eNB allocates uplink grants to the UE without any prior knowledge that the UE has any data to transmit. Typically, a UE can be allocated configured uplink grants for transmission on every subframe. With such a scheme, the UE can transmit as soon as new data arrives in its uplink data buffer, thereby reducing the latency of uplink transmissions. In such cases, most of the time, the UE has no uplink data in its buffer and as a result, it would send only padding transmissions on PUSCH.
Skipping sending useless PUSCH transmissions would be advantageous because it would decrease uplink interference and improve battery efficiency. However skipping PUSCH transmissions may raise operational issues within an LTE system and the present invention proposes methods and devices which take account of such issues.
In legacy (current) LTE UL (uplink) scheduling, there is no possibility for a UE (User Equipment) to skip PUSCH (Physical Uplink Shared Channel) transmissions. New transmissions are scheduled through dynamic or configured (i.e. periodic) grants whereas HARQ retransmissions are scheduled through dynamic UL grant (adaptive retransmissions) or PHICH NACK indication (in the case of non-adaptive retransmissions). Moreover, there is no data-associated UL control information (e.g. TBS, MCS, HARQ process ID, NDI, RV) as this information is basically indicated by the scheduling (or implicitly derived). An eNB (or eNode B) has all the UCI required to process the UE transmission. An eNB may be configured to always assume that the UE has transmitted the PUSCH as intended, and use received PUSCH to update power control loop, adaptive modulation and coding (link adaptation), or timing advance.
However, if a PUSCH transmission is skipped, then the eNB can no longer assume that the UE has performed the scheduled transmission as expected. In the so-called fast uplink access use cases, most of the time a UE would have actually not performed any transmission. Hence, PUSCH transmissions in such cases can be considered as “unsolicited” (meaning: the eNB does not know that there is an actual PUSCH transmission), whereas legacy PUSCH transmissions can be considered as “solicited” (and mandatory-the eNB knows that it can expect a PUSCH transmission). Putting the possibility to skip PUSCH transmissions into effect in a legacy LTE system would have the following impacts.
Firstly, consider HARQ (Hybrid Automatic Repeat Request). The eNB may not be able to reliably perform PUSCH DTX detection. This would lead to the soft buffer being corrupted by noise samples. A consequence can be that HARQ is no longer possible. A very conservative MCS (Modulation and Coding Scheme) may need to be used.
Secondly, consider power control and link adaptation. In cases of SPS (Semi-persistent Scheduling), the PUSCH can be used to adjust power transmission on a short term basis, while PUSCH statistics can be used to adapt the MCS (coding rate). If a PUSCH transmission is skipped, then these capabilities will no longer be possible and as a result, a more conservative MCS may need to be configured. Periodic SRS (Sounding Reference Signal) could be configured as well. However this is not really adapted to the SPS use case but to frequency-selective scheduling, nor is it well adapted to very small bandwidths (minimum SRS BW is 4 RBs, and starting RBs are multiple of 4). Moreover, it would consume additional resources.
Thirdly, consider the PDCCH (Physical Downlink Control Channel). The absence of systematic PUSCH transmissions also means that there will be no confirmation (from the UE to the eNB) that a PDCCH indicating an UL grant (dynamic, or for SPS configuration/release) was correctly received. Hence this may also impact the downlink aggregation level configuration algorithm.
Fourthly, consider ‘UE lost’ recovery. There is currently (in legacy LTE systems that is), no specific UE-triggered action when a PUSCH does not go through. However, in legacy systems, the eNB can take appropriate action since it has full knowledge of the UL issue. Conversely, when PUSCH transmissions are deliberately skipped, this action to be taken by the eNB would no longer be possible.
Fifthly, consider implicit SPS (Semi-persistent Scheduling) release. The implicit release mechanism whereby the UE autonomously releases the configured SPS resources after N successive UL padding transmissions is no longer applicable.
In one embodiment, the UE is configured with a DTX (Discontinuous Transmission) pattern which is based on a subframe offset, a cycle length and an on-duration expressed in number of subframes, such that a UE is requested to obey PUSCH transmissions requests (even when there is no uplink data) at instants known to the eNB (“on” periods of the DTX pattern) and is allowed to skip UL transmissions (when there is no uplink data) at all other times (“off” periods of the DTX pattern). The offset may be UE specific. This embodiment has the advantages of permitting the eNB to use a legacy adaptation algorithm while still keeping the benefits of skipping UL transmissions most of the time. Padding UL transmissions are therefore skipped only during “off” periods. The DTX pattern may be configured into the UE by appropriate messaging from the eNB. In this way, the eNB knows on which subframes skipping of padding transmissions by a UE is not allowed.
The DTX pattern may be aligned with a DRX (Discontinuous Reception) pattern in order to minimise the UE power consumption and the signaling impact.
In the cases of contention-based (CB) configured or dynamic grants, in addition to a UE specific offset, a DMRS (Demodulation Reference Signal) cycle shift may be applied within a hyper-pattern of the DTX pattern.
In a further embodiment, PHR (Power Headroom Report) is also skipped when skipping of UL padding transmissions is enabled and no UL data is being sent.
The Power Headroom Reporting procedure is normally used during UL transfer to inform the eNB about the difference between the nominal UE maximum transmit power and the estimated power of PUSCH transmission. Typically, a PHR has several possible triggers. One of the possible triggers is the expiration of periodicPHR-Timer, which is used to provide periodical PHR to the eNB. Once triggered, PHR is sent in a TTI (Transmission Time Interval) as soon as UL resources are allocated. In legacy operation, UL resources are basically allocated when there is UL data to be sent. In such cases it makes sense to have PHR sent. When there is no UL data to be sent (e.g., DL only during some time, end of a TX speech burst in VoLTE, . . . ), triggered PHR are actually not transmitted. If skipping of UL padding transmission is enabled, it is not clear if PHR would be sent when no UL data is sent. It depends whether “skipping UL padding transmission” encompasses also transmissions with PHR. On the one hand, this may not be needed as this is not done in legacy systems. On the other hand, this may be considered useful in the sense that it provides periodic UL transmissions which can help in maintaining the UL. For instance, if periodicPHR-Timer is configured, PHR can be sent at least periodically. However, as PHR has no usefulness when no UL data is sent, also avoiding sending PHR under such circumstances provides a further power consumption and UL interference reduction.
In a preferred embodiment, the UE is configured to skip sending PHR when no UL data is sent during ‘off’ periods dictated by the above-mentioned DTX pattern. During ‘on’ periods (dictated by the DTX pattern) where skipping PUSCH transmissions is not allowed, the UE obeys legacy requirements and if granted by the eNB, it transmits PHR, if any had been triggered previously. That is to say that the UE is allowed not to transmit PHR on unsolicited PUSCH when no UL data is sent. This option benefits from periodic eNB-aware UL transmissions and reported PHR.
Similarly, in a further embodiment, periodic BSR (Buffer Status Report) is also skipped when skipping of uplink padding transmissions is enabled and no uplink data is being sent.
The Buffer Status Reporting procedure is normally used to inform the eNB about the remaining amount of uplink data to be transmitted within the UE uplink buffer. Typically, a BSR has several triggers. One of the possible triggers is the expiration of periodicBSR-Timer, in which case the BSR is referred to as a “periodic BSR.” The periodic BSR is used during uplink data transfer to keep the eNB informed about the UE buffer status. As such, it is transmitted only when the UE has uplink resources allocated. If a UE has no uplink resources allocated, it is just postponed and does not trigger a scheduling request procedure, contrary to “regular BSR.”
In a preferred embodiment, the UE is configured to skip sending periodic BSR when no uplink data is sent during “off” periods dictated by the above-mentioned DTX pattern. During “on” periods (dictated by the DTX pattern) where skipping PUSCH transmissions is not allowed, the UE obeys legacy requirements and if granted by the eNB, transmits periodic BSR, if any had been triggered previously. That is to say that the UE is allowed not to transmit periodic BSR on unsolicited PUSCH when no uplink data is sent. This option benefits from periodic eNB-aware uplink transmissions and reported BSR.
A further embodiment takes into account the handling of a lost UE. Typically, this can occur when the UE has moved towards the edge of a cell. It can also be due to the time alignment being lost. In current systems (legacy operation), if a UE needs to transmit UL data, and there are no scheduled resources available, the UE uses SR (Scheduling Request) (if time aligned) or PRACH (if not time aligned). In either case, the UE notifies upper layers in cases of failure. Typically, a SR procedure failure will trigger a PRACH procedure, and a PRACH procedure failure will trigger a radio link failure (RLF) procedure from RRC. As the eNB is not aware that the UE is transmitting, failure handling is important at the UE side in order to solve the issue.
In cases where there are scheduled resources available, the UE can transmit data on PUSCH. The eNB is responsible for requesting HARQ retransmissions when required. However, in cases where the maximum number of HARQ retransmissions is reached on the UE side, there is no specific action at MAC level. The transmission goes on with a new transport block. RLC AM bearers will eventually reach a maximum number of retransmissions, and trigger a RLF. But if there are only RLC UM bearer(s), then no RLF is triggered at all. As the eNB is aware that UE is transmitting in this case, failure handling is less important at the UE side since the eNB can detect UL issues and take appropriate actions.
With configured UL grant every subframe, a UE is always in the “scheduled resources available” use case and does not use SR procedure. Instead, PUSCH is used. If skipping of UL padding transmission is enabled, then the eNB is not aware that the UE is transmitting. Then, if UL is lost, eNB will not take any action since it is unaware of it and as per the current LTE specification, neither will the UE; in cases where RLC UM bearer is used, the UE may go on transmitting until TAT (Time Alignment Timer) expiry. Such timer could typically be set to a high value in order to avoid unnecessary timing advance updates.
These issues relating to a lost UE are resolved by the provision of the DTX pattern as described above. This is because the eNB would notice that the UL connection had been lost by no longer decoding solicited PUSCH on mandatory occasions.
In an embodiment where only unsolicited UL PUSCH transmissions are possible; that is, the DTX pattern is not configured in the UE, and the UE is allowed to skip uplink padding transmissions on an allocated uplink resource, then the UE may be configured to autonomously release any “skipping UL padding transmission” enabled resources upon noticing that the PUSCH transmissions are not successful. An option is to configure the autonomous release to occur after a given number of unsolicited PUSCH transmission failures. This would result in a reversion to the legacy operation involving SR, PRACH (Physical Random Access Channel) and RLF procedures. Reception of a SR/PRACH transmission by the eNB may serve as an indication that the UE has released the SPS resources. The eNB can then recover the situation and allocate appropriate resources to the UE.
In other cases where dynamic grants are used for pre-scheduling, it is assumed that this would not be on a per-TTI/long term basis. Hence, the issue of unsolicited PUSCH transmissions not seen by the eNB may be less critical. Indeed, the UE can be expected to still send some SRs, which could be detected by the eNB. In one embodiment, the UE may be configured to ignore at least some UL grants and instead, send (SR) Scheduling Requests, at least until detecting a change in MCS/power control in the UL grants. An eNB receiving a SR on TTIs on which an unsolicited PUSCH was scheduled (hence on which the UE should not have transmitted SR) can interpret it as an issue with unsolicited PUSCH and take appropriate actions.
The way that a UE can determine an unsolicited PUSCH transmissions issue depends on an agreed scheme for HARQ feedback. PHICH is strongly encoded, and the NACK-ACK errors are expected to be low. Alternatively, ACK feedback through PDCCH can provide more reliability in the determination that PUSCH transmission was successful.
The UE may be configured to autonomously release the configured resource after a specified duration of time has elapsed (or number of resources) since the activation.
It will be appreciated that the implicit release of SPS resources after N successive UL padding transmissions in legacy systems will no longer apply if PUSCH transmissions are skipped. Having a mechanism to implicitly release the SPS resources could also be useful to mitigate UL lost issue impact. It should be also noted that once SPS is configured by RRC, it could be accidentally started on a false PDCCH detection, in which case the eNB is not even aware a UE is transmitting (on a randomly selected resource). At least in legacy operation, the configured resource would be released once UE has no UL data to send. However, this would not be the case if PUSCH transmissions are skipped. A possible solution would be to limit the number of pre-allocated resources to a given amount M (fixed or configured), or equivalently to release the configured resource after M successive periods.